Microlithography is used for producing microstructured components, such as for example integrated circuits or Liquid Crystal Displays (LCDs). The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the extreme ultraviolet (EUV) range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process. Such EUV mirrors comprise a substrate and a multilayer system arranged on the substrate for reflecting the electromagnetic radiation impinging on the optically effective surface. A highest possible reflectivity of the individual reflective optical elements is desirable in order to ensure a sufficiently high total reflectivity.
In order to ensure a highest possible radiation throughput in a microlithographic projection exposure apparatus, it is endeavored, in the case of high local angle-of-incidence bandwidths, to reflect all rays of the local beam as uniformly well as possible at the individual reflective optical elements. For this purpose, the number and thicknesses of the individual partial stacks (i.e. the “period lengths” of the individual periods) of the multilayer system are optimized. In the simplest case, periodic multilayer systems can be involved, that is to say multilayer systems having substantially identical partial stacks, in which the number of periods is reduced to an extent such that the reflectivity curve has the desired width, wherein however the reflectivity still varies greatly with the angle of incidence and the wavelength.
In a further step, the multilayer system can also have two sections, in which the respective total stack thickness and the layer thickness ratio within the stacks are different. Furthermore, these two sections can also have different numbers of stacks. In variants, three or more sections having different total stack thicknesses and layer thickness ratios can also be provided. A further approach consists in totally canceling the boundary conditions for the thicknesses of the individual layers. This leads to a totally stochastic, also referred to as an aperiodic, multilayer system. In this way, it is possible to design the most flexibly multilayer systems whose reflectivity varies as little as possible with the angle of incidence and the wavelength. One feature of such stochastic multilayer systems is that numerous layer thickness sequences can result in very similar reflectivity curves both depending on the wavelength and depending on the angle of incidence.
For the performance and the optical properties of a reflective optical element which is used together with further reflective optical elements in a microlithographic projection exposure apparatus, the lateral profile of the individual layer thicknesses, which is also called profile, is also of importance in addition to the vertical construction of the multilayer system. For controlling the lateral profile during the production of a reflective optical element, it is possible to use X-ray diffraction, for example, wherein the reflectivity is measured depending on the angle of incidence. In this case, the copper Kα X-ray wave line is appropriate for reflective optical elements for the EUV wavelength range. A particularly accurate characterization of the lateral profile of the layer thicknesses is possible if the diffractogram has a sufficient number of sufficiently sharp peaks.
With regard to the prior art, reference is made by way of example to US 2010/0239822 A1.